[mr beer]

First thing is there is no canon answer to this.

But I have considered this question myself and my personal approach would be to work this out in terms of how much stronger than an average man the character is for purposes of lifting weights. This makes the whole thing extremely easy.

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Let's say we think our untrained man can bench 60 kilos. For example purposes, we want our character to be able to bench 150 kilos and his other lifts are in proportion to this - this is key to simplifying this process.

So he is 2.5x stronger than the normal man.

In GURPS, a normal man has Basic Lift 20. Our guy needs to have Basic Lift 50, because he is 2.5x as strong. Which means he has ST 16 (Basic Lift 51).

Now that's not all pure ST, he should have Lifting ST as well. The usual guide is you can have a couple of levels of Lifting ST, so let's give him Lifting ST+2.

Now our character has ST 14 and Lifting ST +2 and he can lift 2.5x as much as a normal man for all our standard lifts. You can decide what these values are. If you write down what you think a normal man can lift, you can simply multiply by 2.5.

This works well enough for game purposes, unless the campaign is all about competitive weight-lifting or bodybuilding contests.

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You can fine-tune this by reducing HP or Striking ST by a point or two if you like.

I would skip Arm ST because then that means you need to consider which lifts are Arms and which are not - simple when we consider military press and squats, but not so much with say the deadlift.

I wouldn't use the Lifting skill at all, because it's a terrible way to model real world weightlifting. Alternatively you could re-work the skill to say add +5% to the amount you can lift per point of skill above 10 or something. I'd just skip it altogether though.

I think that this approach passes a sanity test as well. In GURPS, normal ST is 10 and maximum ST is 20, which implies that real world strongest possible men are 4x as strong as normal people. Which is ballpark correct.