You need to check the scoring w/ last year's stats. Most common mistake IMO for late picks is picking by position, like RB.
If Peyton, for example, should have gone at 6 and he's still there you must take him. You can't leave him on the board. Must is a tough word for FF but, at the end of a round you cant let someone swing into the next round if he's a very good pick.
Predrafting the whole draft is tricky
I map out a whole ten rounds using last year's ADP to guess when guys will go.
Might be RB, RB, WR, WR, QB, TE or somesuch
I'd start making your WR in round 3 list. I've found that's a better starting point than round 1. Starts off all the thought processes of who might be there, who might fall, who is too early etc. right away. Never goes smooth for me if I start with round 1.
So to decide on say my WR in round 3, I'll list 10 players. 1 WR from round 1 for giggles, 3 WRs from round 2 just in case, then 6 that should go in the 2nd or 3rd range. NEXT I'll put in some RBs and QBs that "shouldn't" be there. Maybe 5 RBs and 2 QBs. 17? Yep
Don't forget though those 17 adjusted a smidge can be your list for round 2 and 4. Maybe it's 10 RBs and 5 WRs in round 2. Ya got some all done.
Also, predraft some vets in later rounds. Lean toward them moreso than rooks and 2nd year guys. Derrick Mason vs Laurent Robinson for example. Bruce vs James Jones. By the time the season comes, the vets could be coverring an injury or FA traded to be a backup or somesuch and they can be extra beneficial in these drafts