My take on it (purely based on observation) :
Higher Grade affords higher Interdiction Range.
Higher Class affords interdicting Ships with far higher mass and generally make it easier for the Interdictor/more difficult for Evader.
(permits less accurate Interdiction minigame while either still gaining blue bars or not losing much when out-of-position).
Velocity seems to increase interdiction range (got interdicted by an NPC during a Hutton run from well over 3000Ls range while both at >500c )
Starting the Interdiction at very close/minimal Range seems to make it alot more difficult for the Evader. It appears the vertical/lateral vectors developing become much larger due to the physical proximity.
If managing to physically pass the Target, it can cause automatic FSD fail of the Target (as the escape vector exceeds 90deg off during the pass).
So establishing teather at far higher velocity and close to overtaking the Target seems to be a valid tactic (although I'm still convinced some guys cheat and trigger a Network crippler at that exact moment to achieve that result

)
Move 1Ls up when you're only 0.5Ls away creates a far larger angular error than doing the same from 100Ls away, making it additionally more difficult on the Evader.
Disclaimer though : I'm not an experienced Interdictor myself. 100% sure there's more qualified guys around with tons of more experience.