I initially didn't have a front tire leveler.
The generator works fine without it, but I didn't find it comfortable after a while of always pedaling while going "downhill".
Depending on the Bucket height, tire outside diameter and your personal preference, spacing will vary.
First off a word of caution when mounting the bike when it is situated on the stand:
** If you move the center of mass too far sideways, the bike will lean to that side. It will do no good for the axle extender, axle receptacle and the axle itself if you really try hard to point load with all your weight the outboard end of the axle extender by having the bike banked sideways. Keeping the center of mass towards the middle of the bike is far easier to learn than learning to master riding a two wheeler. What you don't want to do is have your body far outboard and step on a pedal kind of like you would do to get on a horse. If you are careful you can stand on a pedal and throw a leg over, but I find it easier to mount the bike from behind by first stepping with my right foot on top of the axle extender on the starboard Bucket (remember later on in the build it will get wrapped with Friction Tape) and then throw your left leg over.
For someone who is not very limber, using a single step stool is also a viable alternative. Just mount from the side, you are up higher than the ground and throw a leg over. See under the "Options" album the one I found at Target which offers good quality at a reasonable price.
Once you learn how to mount and dismount, now you want to figure out how much spacing you may want under the front wheel.
Take some 2 x 6s, 2 x 4s or 2 x 3s you may have hanging around and begin stacking them under the front wheel and go pedal on the bike for a while. On my 26" mountain bike I am using, I found two stacked two 2s together gives me the perfect height I want.
I will describe how to make a more permanent front wheel spacer in the next. pic.