Joan (of Arc)s litter are now 6 weeks old

They are adorable.
They are saddlebacks (
Essex) and I am using the Berkshire Pig website food and weight guide....
I have a pig tape which starts at 60cm which is 37kg liveweight.
According to the Berkshire chart the piglets should be between 11.5 and 16kg now to be on track.
using the tape they are between 51 and 54 cm in girth. Does anyone know what this converts into in liveweight. Most charts seem to start at about 60cm
Any help would be appreciated
