Anyone else finding it pretty difficult to get accurate weightings for LBs?

Got a good enough guess of what they *should* be, speed is just so dominant it's screwing everything up - so getting plenty highly rated mid speed guys.

It feels as if I should just set every LB stat to 1 and speed to 100!

Weights aren't necessarily the problem with LBs. It's understanding which LBs get used where in the different schemes.

For example, in a 3-4 defense, your SLB is going to be primarily an edge rusher (and doesn't line up on the strong side FWIW) and your MLB is going to be your tackling machine. Your MLB2 covers the 5, which means he gets the RB or the FB depending on the set, and the WLB gets the 4 (usually) which means he gets an RB or the TE depending on the set. So you just go across the positions and set your best pass rusher as the SLB, your best Zone+Tackling guy as the MLB1, your 2nd best M2M cover as the MLB2 and your best M2M cover as the WLB.

In the 4-3 defense, your WLB lines up in the right position and ends up being a tackling machine because of the number of weakside blitzes that you will inevitably use against the run. Otherwise, he takes the 3/4/5 depending on which defense you call.

Here is a list of player weights that I have found to use to meet NFL weight player standards:
QB-220, RB 217, FB 243, TE 257, WR 198, LT 312, LG 308, C 283, RG 316, RT 319
DE 276, DT 300, RDE 276, SLB 241, MLB 245, WLB 237, CB 191 SS 207, FS 206

If any of these are not correct, please private me ASAP so I can make the changes so my players can make weight. Thanks.
I have added "check player weights" before pre-season, to my team checklist, so I know who to deactivate until they make weight.
Thanks for everyone's help in this matter.

There is some wiggle room. Weights are specified in the league rules thread.

In general, overweight isn't an issue, underweight is, that's the generalised rule I go by.