Saracen Sahara frame size help

[Turbo12]

Hi all. I've just been offered a very tidy 1993 Saracen Sahara, bit I need a bit of help with the frame size. The Saracen catalogue from that year says they measure their frames from BB to seat tube level with centre of the top tube. So it's a 19" frame by their standards. Normal measurement to top of seat tube makes it 21", which at 5'11" is a bit big.

So, do I go with their measurements, or convention?

Thanks!