The formula is quite simple if your goal is to strictly be a FF'ing machine.
Tck = 68 naturally or higher
Str = Agi = 68 naturally
Monster Hit - at least 5
Power Tackling - 15
Unfortunately, not many people really want this type of build, but could compromise in some areas. My build that I posted in another thread works for a player that wants a LB that can create some fumbles, but not be a machine.
Str = Tck = 61
Spd = Agi = 68
Monster - at least 7 (has to be higher due to lower tck'ing)
Power Tackling - 15
What is the most important skill in forcing tck's? Tackling is. While Strength and Agility play a significant role, the tacklining skill level and being on Power Tackling style contributes more.