Lets say you medium sized river and walking pace push of water, what sort of depth would you expect with 5ft and 10ft poly leader of various sink rates?
I’m afraid that this is one of those ’length of string’ questions because there are so many variables:
The density of the fly, as I described above
The nature and length of the tippet material - Maxima is buoyant, fluorocarbon sinks
The angle of your cast relative to the flow - the more square the cast the more it may sink before hydrodynamic lift comes into play
Whether or not you strip in line
Whereabouts in the swing you measure the depth - it won’t be the same throughout
Thus a sparsely dressed fly on the end of a 5 foot polyleader and 8 feet of Seaguar cast square and not stripped may fish up to a foot deeper than a bushy fly on a 5 foot polyleader and 8 feet of Maxima cast at 45 degrees and retrieved with a brisk figure of eight.
As Rennie noted above, in the face of so many variables you can only experiment and apply judgement.