This is sorta complicated.
10% of a PC's mass is his weight allowance, i.e. how much he can carry without encumberance.
So lets say out PC is 200 lbs (90kg in metric). That means he can carry 20 lbs with no penalties. For every additional 20 lbs, or fraction of, he suffers a -8 encumberance penalty to all movement mnvs, and his pace will eventually be limited also.
Weight penalty can be off set by St modx3. So if he has a St mod of +6, he will off set encumberance penalty by +18. Note: if PC has a negative St mod, assume it is 0 for calculating enc penalty.
So, our PC's weight penalty's look like this;
0-20lbs; -0
21-40lbs; -0
41-60lbs; -0
61-80lbs; -6
81-100lbs; -14
101-120; -22
121-140; -30
141-160; -38
etc
By calculating the enc penalty to -100, you will have the weight were the PC simply cannot move. The weight lifting skill can allow a PC to move greater weight easier than a straight Stx3 mnv roll (because the skill provides a higher bonus than Stx3 obviously).
It is easy to generate a table that shows weight to St mod for enc if you don't want to calculate it every time.
Also note on the enc table is a limitation to pace based on total enc penalty.