Too much imo. Doesn't take into account hassle, late payment deductions of sfp. Pay them 75% max of that. Having said that I think your formula is the only way you will compete in a lot of circumstances.
Agents won't take into account that either. I bid £130 an acre for a 96 acre farm, with 4 bedroom house. The farm was a ruin, buildings rundown, no fencing and pasture knackered. The view of the agents was that its market value was significantly higher and I wasn't even invited for interview.
Money is the biggest playing, no matter what anyone else says.