I don’t have the answer to your direct question about targeting an overall shader complexity, but I will confirm for you that Forward Rendering bumps the numbers up quite high and Deferred Rendering makes the number lower. That is to be expected.
Keep in mind that you can have a small portion of pixels on the screen have super high instruction counts (a glass window or fancy effect) and you won’t necessarily kill performance if everything else is in check. Do some performance profiling with what you’ve got, make sure your shader complexity is your bottleneck (like maybe you have too many draw calls and the GPU is just sitting around waiting).
Some common hacks for VR performance include making everything Unlit and faking the lighting inside the material. You can also hit the checkbox for Fully Rough in the material to take that out of the equation. Don’t use Two Sided.
Hopefully some other mobile VR devs share their average shader instruction counts to help you feel more confident about what you’ve got.