Lucky, what do you get for 6-7 litres per hour I wonder ? For 2 and a bit litres my Toro does a 42" cut at about 11kph; uses a bit more in heavy work.
My guess is it is better to put the petrol in at whatever it costs in your $ costs list, and put the time you spend fuelling into the biggest grab bag of them all, ADMIN TIME !
Loading in the morning (if like me you have to load and unload each day)
Unloading at night
Shopping - for petrol, parts, new machines, fluoro shirts, work boots etc. etc.
Keeping records, drawing up work lists etc.
Ringing people, texting, emailing
Travel time between jobs
Travel time for emergency repairs
Travel time to tip
Time spent participating in forums to keep in touch with what's going on in your trade
Time spent on staff if you have any, hiring, firing, co-ordinating, supervising, talking to them
Time spent chatting to lonely old customers
Admin time is probably 1 hour for every 3 you work ... so if a labourer's pay is $25 ph, add $8.
(This $8 is distinct from what you deserve extra for being the businessman who makes things happen, the risk-taker, decision-maker, inventor. Running a little mowing business isn't quite as complicated as running Qantas, so don't allow $5,000 per hour for this, but maybe $10 ph?)
If you take my $20 per hour for machines, $25 for labour, $8 for admin time, $10 for extra pay for being the boss, you get to $63 per hour as the minimum you should reasonably charge. And this is being (according to me) super conservative and trying to avoid exaggeration. And then you hear of people charging $40 per hour