3x5 index card. Decide on the hackle length you like (1.5x, 2x, etc), lay the hook on the card with the point along the bottom edge and make a mark along the shank. Move the hook point to the line and make another mark 1/2way up the hook gap, along the shank again (for 2x length) or whereever you want. Repeat for each hook size you want so you end up with staggered lines. keep the card handy and just slide the card into the hackle to find the right feather.

Pretty cheap and you can make up different cards for different style or brand hooks.