"Ppem size out of range" error with relative inheritance

Posted by: TomTom

Remember the "Ppem size out of range" error when using nested CVT inheritance? That raised some confusions and discussions, since the reason for thatis more mathematical than typographic - but at the end: if you get it, you maybe really did something wrong!
Now the latest versions of VTT introduced relative CVT inheritance. This is a nice feature and makes some things more easy and more logical to handle. The cut of PPEM gets more real, since it is the exact value where the distance changes from 1 to 2 pixel.
But although absolute and relative inheritance handle things very different, the "Ppem size out of range" does not make a difference between them.

Consider this:

AnyGroup
AnyColor
AnyDirection
AnyCategory
65: 96
X
66: 125 = 65 @ 25
Black
UpperCase
X
StraightStroke
68: 135 = 66 @ 80
RoundStroke
69: -6 ~ 68 @ 60 <- You will get an error here!

The straight stoke follows the main stroke very long, since there is not a big difference between them. The round stroke is relative to the straight stroke, and needs to be 2 pixels at 60 ppem.
How can I solve this? Make the round stroke relative to the main stroke? This would be correct for absolut inheritance, but not a good idea for relative inheritance!
Is the "Ppem size out of range" error really correct in that case?