You only need a few inches to hit the G spot and that should be sufficient to get her pussy tingling and her juices flowing. If you want to hit her A spot then that's a different problem all together.
Suggest you read up on Master Mantak Chia - the screwing technique is probably the most fundamental of sex techniques yet MOST guys still don't fucken get it..as Mantak Chia says, learn your techniques and master them.
https://m.youtube.com/watch?v=lZFEW657vbw
Btw: Good post James_Jones !