Kerbweight

There is nothing wrong with using kerbweight as a basis for calculating the weight ratio, but kerbweight is not documented anywhere so if there is a need or a desire to have a really accurate figure the only way is to establish the kerbweight by putting the vehicle on a weighbridge.

On the other hand, definitions of terms such as kerbweight, mass in service or mass in running order don't differ that much that using one or the other instead would result in a significantly different figure. Whether the calculated weight ratio comes out as 82%, 85% or 88% is not going to make any noticeable difference to the stability of an outfit. I would challenge anybody to be able to tell the difference. It's not all black and white, there is a big grey area in between.