The distance between the mounting eyelets on the rack and the ones in the frame is about 8," while the rack-to-seat post distance is a little less. There are struts (attachment arms) for sale which are 11".
I plan to carry close to the 40 lb maximum. Considering the weight, would you recommend mounting the rack to the frame or the seat post? (As a city hybrid, this set up is a downgrade from the stronger, heavier set up on my touring bike.)