well... consider this.
being a marine is a rather standard job. there aren't that much career choices there. and a level 10 doriki means mostly an average dude with a weapon. doesn't even have to be fit or strong. in any mandate army you'll see lots of guys like this, who are effectively meatshield, if things ever came down to rumble.
Spandam also seems more like a 'strategist' and less of a fighter. so a level of 8-10 indicates a merely healthy person. not a level of supposed strength.

so we have Ussop, who's supposedly power level is 10. like any other person.
he's a carpenter, which is a demanding job. (X2)
probably trains on a regular basis (X2)
took on hits from merman, said to be far stronger than human (X2)
entered the grandline, which is said to have harsher physical conditions then the rest of the world (x2)
same with the mole woman (X2)
visited sky island, which is training in low oxygen setting (X2)
got hit by Enel's lightning, which probably gave him some sort of super powers (X2)
Zombies! (X2)
so far..
10x2x2x2x2x2x2x2x2.
that's like 10x2*8 = 2560 doriki! same as Zoro!
well, obviously not.
I'm sort of lost my train of thought with this, but still, if a marine soldier is a weak as he appears (and they probably are that weak), then Ussop is at least 100 doriki.
if not well over 9000!!!!!!!!!!!!!!!!!!!