The fact you are getting 15% variation tells you something is really really wrong so it's hard to work out what to say about your setup. If a torch light changed by 15% while you turned around you would clearly see for example so it's obvious your result is badly wrong.
Using your own crazy logic the speed of light is 300000000 m/s the speed of earth is 30000 m/s (30km/s) so the difference using you crazy logic should be 30000/300000000 = 0.001 or around 0.1% assuming the photons bunched up to increase the brightness .... I think that math's and your logic is right.
Here let me give you some hints about what is known:
http://en.wikipedia.org/wiki/Ring_laser_gyroscope
A ring laser gyroscope (RLG) consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the internal standing wave pattern in response to angular rotation.
These gyros are used on most planes and many ships because they have no mechanical parts.
They work because of Sagnac effect
http://en.wikipedia.org/wiki/Sagnac_effectYou can solve the Sagnac effect in your head either classically or via special relativity either mathematics gives you exactly the same result
At non-relativistic speeds, the Sagnac effect is a simple consequence of the source independence of the speed of light. In other words, the Sagnac experiment does not distinguish between pre-relativistic physics and relativistic physics.
So do you understand ... Special relativity (Einstein) or not sagnac gives us the value of the deviation.
I can tell you it is nothing like 15% the exact amount depends on a whole pile of things about the setup like frequency, temperature, media etc but it's a percent or two at most.
If you look at the size of a standard gyro device it tells you even on small size if you are playing with light you can't ignore the effect
The key part about sagnac is you can solve it correctly without needing special relativity however because this is the case and many ignored special relativity something important gets missed the Wikipedia article picked it up
The Sagnac effect has stimulated a century long debate on its meaning and interpretation, much of this debate being surprising since the effect is perfectly well understood in the context of special relativity. An essential point that has not been well-understood until recent years, is that rotation is not required for the Sagnac effect to be manifest. What matters is that light moves along a closed circuit, and that an observer is in motion with respect to that circuit. In Fig. 5, the measured phase difference in both a standard fibre optic gyroscope, shown on the left, and a modified fibre optic conveyor, shown on the right, conform to the equation Δt = 2vL/c2, whose derivation is based on the constant speed of light. It is evident from this formula that the total time delay is equal to the cumulative time delays along the entire length of fibre, regardless whether the fibre is in a rotating section of the conveyor, or a straight section. In addition, it is evident that that there is no connection between the total delay and the area enclosed by the light path. The equation commonly seen in the analysis of a rotating, circular Sagnac interferometer, Δt = 4Aω/c2, can be derived from the more general formula by a simple substitution of terms: Let v = rω, L = 2πr. Then Δt = 2vL/c2 = 4πr2ω/c2 = 4Aω/c2.
This sets up the good old extended conveyer belt example which is the same as your line in an arc simplification you want to make
So in answering your question .... NO you can't ignore the effect even if you want to argue the arc movement is large and you are basically describing a straight line as an approximation because the effect still occurs in a straight line movement.
Now as I said these things are notoriously hard and catch even scientists out so no one in the science world is going to pay one bit of notice to you especially given the other evidence from multiple fields that special relativity is right.
What I would add is that at low speeds I personally doubt you could separate classic physics from special relativity they will always give the same result, sagnac sort of shows that .... your experiments assume you can but you don't properly understand special relativity. The real difference comes at very high speeds like in particle accelerators, the atom and QM when classic physics gets it all wrong and clearly wrong ... so I guess what I am saying no Nobel prize for you
