Personally, I like to mock things up with stools, planks, c-clamps, etc. and use the resulting measurements to figure out the ergonomics. Usually the designer gives guidance on this, but since you designed it yourself you're on the hook for this :-)

Unless your boat is pointy at both ends, you'll have to make sure that your weight is forward enough that you won't drag your transom (that's a real speed killer). The best way to find the right seating point is to put the boat into the water using a box or stool or something as a temporary moveable seat. Move it back & forth and have a friend watch and tell you when the boat is properly balanced. If the seat placement ends up ridiculously forward, you'll have to put the seat somewhere sane and put ballast up front.

The position of the oars along the length of the boat have to do with stability. If the oars are too far back the boat will be negatively stable and try to wander. If the oars are too far forward the boat will be positively stable and track like a freight train on rails and never want to turn. If they're at the sweet spot, the boat will be neutrally stable and be wonderfully maneuverable yet be easy row straight. This position is affected by weight distribution, the shape of the underside and the "sail area" of all the stuff above the waterline (including the rower).

The final piece of the puzzle is the oar length. That also depends on all the stuff above, as well as the rower's strength. Short of knowing enough to precalculate all the forces involved, the best thing is to make a guess and try out some temporary settings with moveable stools and c-clamps for oarlocks on the water. Once everything is right, mark the positions and make the permanent installations.

Here's some links to Jim Michalak's thoughts and experiences on all this. Most of his boats aren't my style, but his oar/rowing discussions are good stuff based on good practical experience. There's some design work and some woodwork here (with pictures).

http://www.boatbuilding-links.de/Jim-Michalak/oar-building-and-rowing.htm

http://www.jimsboats.com/15jul09.htm

Good luck,

Laszlo