It's strictly proportional - both the mission scripts and the MissionMatching.gf settings operate by flat ratios. The missions set a minimum and maximum ratio for the drafted ship's BPVs, and those get multiplied by the MissionMatching.gf ratio.
For instance, if the mission says draft an enemy ship between 0.7 and 1.2 times the player BPV, and the MissionMatching ratio is set at 1.1, then the actual ship drafted should be in the range 0.77 to 1.32 times the player BPV. (Note drafting cannot account for fighters/PFs - it strictly uses the shiplist BPV for a ship.)
Of course, for a given year and a given ship, most of the "in range" enemies might be clustered at either end of the specified drafting range, creating either sweet spots or death traps at any given time.
Believe it or not, the ED missions, with only a couple of exceptions, use lower bounds of around 0.65 to 0.75 and upper bounds of 1.1 to 1.3, (before any MissionMatching.gf factors get applied) Though some of the nastier missions do specify 3v2 odds against you in enemy space.
dave