The 100mm of travel is at the rear wheel. The leverage ratio of the bike based off pivot placement, chainstay length, etc detirmines the relationship between the shock stroke and the rear wheel travel. In other words, it's perfectly normal.
And on the sag, you measure that off the shock stroke. The shock doesn't use the full shaft. I would guess your shock has a 1.5" stroke since that's a common size close to what you measured. So with a 1.5" stroke unweighted, it should sink about .3" (a hair under 5/16ths of an inch). That should be close. If you're used to a hardtail, you may want to go a bit firmer at first to ween yourself on to full suspension or you can jump right in and let the suspension work like it should.
Front shock sag setup works the same way only it's 1:1, so about 20mm sag.
And on the sag, you measure that off the shock stroke. The shock doesn't use the full shaft. I would guess your shock has a 1.5" stroke since that's a common size close to what you measured. So with a 1.5" stroke unweighted, it should sink about .3" (a hair under 5/16ths of an inch). That should be close. If you're used to a hardtail, you may want to go a bit firmer at first to ween yourself on to full suspension or you can jump right in and let the suspension work like it should.
Front shock sag setup works the same way only it's 1:1, so about 20mm sag.
