That sounds about right, length wise. Try not to use a solid object like a pen, as it will not curve to fit. Try using your first and second finger instead.
! finger or about 10mm space would be considered a race fit (ie very tight high performance fit), 2 fingers, about 20mm would be what most people go for. It would be a snug fit without being overly tight.
Dont forget though its good practice to ensure your foot is in a neutral postion when doing a shell check. If your foot is pronating alot, chances are your foot will be longerthen it will be when you have a footbed in the boot. This is why often I make the footbed first as it gives a much better shape for doing the shell check.
All those little things are what you need to look at, it could be that the hee cup is the wrong shape for you, even though the length is ok. Its all these little things that boot fitters deel with every day, so tahts why its always recomended that you go to a good fitter.
People often just think its length and width, but there is so so much more, thats why it takes years to get ok at bootfitting.
For you, I would take them to a bootfitter and have him asses them. Should not cost anything, but there is so much that goes into it, it would be inpossible to go over it all online.