simple geometry/trig help for CAD

[pomprocker]

I am trying to do a google sketchup of this right here:
http://members.cox.net/rbirac2/Ebot/EbotImages/Ebot_base1.jpg


I'm trying to do the rounded edges of the wheel wells but I cant seem to figure out where to place my circle/arc it seems when go in to the specified radius of 3/8" I get a circle that sticks out past the corner. like this:



edit:

I guess the radius isnt centered from the corner, but i can't tell where I should put the circle center.

[pomprocker]

nevermind i figured it out, instead of drawing a diagonal 3/8" from the corner, i drew down both sides, and then straight across until they met, i don't know the math for this, but I wish I did.

[airman00]

btw sketchup finds the center for you automatically 

move the pencil tool cursor around where it looks like the center of the circle is and then you'll see a point turn green. Thats the center point