Trial and error. Measure the water in:- a little less each time until you achieve the desired result.

"The main thing is that the cooker must never be allowed to boil dry, the consequences would be disastrous."

It's not a perfect world and, if you are anything like me, one day you may get it wrong let it boil dry in the middle the pressure cooking. You will smell it, believe me, so stay with it in the kitchen, especially if experimenting. It's a good meal wasted and a good deal of hard scrubbing to clean the burnt food off the pan but the cooker should be recoverable if you catch it as quick as you can. It will set your lower limit, so it's a result of sorts, though not one to be recommended.

"Actually you do lose water during the cooking, unless the design of pressure cookers has changed fundamentally from that of my 30-year-old Prestige, that is. It escapes as steam when you're up to pressure, hence the hissing noise from the spring- or weight-controlled valve."

The amount of water you lose will, of course, depend on how high the heat is once the cooker has come to pressure. It sounds as if your mum left hers turned up high. I turn mine down until it is only just hissing.

DC