because when I negotiate a monthly allowance (my normal pattern) I quote them figure "x" but then say with their and my busy schedules it would be unlikely we would meet more than 60% of the scheduled time... so I accordingly discount the original monthly allowance by 60%.  
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So far the "logic" of such an arrangement has passed SB muster nearly every time.

I don’t do monthly or weekly allowances. I travel too much. When the subject has come up in negotiations, I tell a pot SB that I’m interested in that I plan to meet them 2-3 times a month. I have lost a few who say that won’t pay their bills but that’s not my problem. Most have been good with that and I pay them per meeting.

during initial talks about visitation frequencies.  This has seemed very acceptable to the SBs, although some with whom I have been unable to close the sale may have expected greater frequency.  In practice, twice a month only works out occasionally.  I've seen one or two more frequently, but most less often.

I may be in the position of having to juggle multiple good options for the first time.  I look at is as the feast offsetting extended famines.  I'll manage, somehow.  

...upon initial meeting I tell them that,  if EVERYTHING works out GREAT, we can meet 2 or 3 times a month...but rarely does everything work out quite that well so twice a month is about average for even the REALLY good ones.
Occasionally after the first month or two they begin to complain a bit or start hounding me for dates and then I put them on the back burner for a while until,they either cool off and play nice or move on.
I also enjoy having a long distance arrangement with a special lady that I can see once every 2 or 3 months to do some travel together for weekends or longer.
I like to keep some time and my options OPEN for chasing new pot SB's !

The caveat is:  a. If wife or kids are home from work/school all bets are off.

The reality is that due to holidays, teacher conferences, flu, doctor appts and simply the wife playing hooky on a Friday, I don't usually get 4 in a row anyway.  Plus, the cash saved that week goes to my TER provider fund for ocassional ATF action.

So far, no problems.

I always tell a prospective SB that we'll meet 2-3 times monthly.  That is quickly followed by how busy I am with work, family, travel and social/civic activities.  In reality, I aim for 1-2 visits every 6 weeks with all but my MVP.  I see her whenever she wants which is about twice monthly.