I am a small company, started full time in May 2010. At the end of that season a friend told me about level billing. Pretty much what you said, take total amount and divide by 12. That has worked great and supplied me 12 month income. As I have grown I still like it but the down side is the hrs you and your help put in during the season. And yes full service; cut, trim, edge, blow, hedge trimming, bed maint, mulch/roch top dress, plant install, color change, etc.
It took til this year for me to figure out the money cycle on level billing.
Being that I had alot of financial responsibilities(kid, house, food) I was selling everything from gutter cleanings(a lot), Christmas Decorating(none),
window washing(none), parking lot clean ups(two). One of the busiest things I sold(a big pain) was clearing out brush and overgrown chain link fences(3). The work is hard but it was a money maker. I do like those jobs in the winter because of less bugs, no leaves and if that is your only job for the week, slow down.