Now I’m no owner-driver but common sense tells me you don’t want it on a ppm basis if you aren’t doing distance.
If you are doing distance then I think £1/pm is being optimistic for an 18 tonner. Dunno, try it and see.
If you’re doing locals then you need to agree a fixed rate between A and B, taking into account waiting/unloading time which you obviously can’t charge for unless it’s excessive.
Cheers.