Meaning...if my saddle rail position is dead center on a 20 degree setback post, will I have to slam the saddle back beyond spec to get the same position?
How I manage this is I add the saddle setback to the Effective Top tube length of the known frame setup and then compare that to the frame that has a zero setback seat post.
For example, your Tavelo Attack frame size L has a 15mm seat post setback and an effective top tube of 553mm (390mm reach).
Your total
"Imaginary" effective top tube length would be
15+553 = 568mm.
To translate that to the VB-R-268 frame that has a zero setback seat post, you would need to go with the size L of the VB-R-268 which has an effective top tube of
563mm (398mm reach). Then adjust stem length according to your needs.
Coming from a 20mm setback you may need to push your saddle about 5mm forward, off dead center which is fine as far as you do not go beyound the saddle's safety mark.