That doesnt make any sense either.
So I go from 20k + 10k subsidy to 25k + 10k subsidy, to go from 30k to 35k total. Fine.
Someone else might go, by your logic, from 15k + 15k subsidy to 20k + 15k subsidy to 35k total.
SO you have this wierd situation that your subsidy is whatever it was when you first started getting it, and never changes.
SO if I start work in my first job on 35k, I get no subsidy, ever.
Someone else starts on 10k and gets a 20k subsidy that they retain in perpetuity to ensure they benefit from any pay rises.
So even if the second guy overtakes the first , he'll still get his bonus?
I'd suggest that you havent really thought this through.....