The idea of calculating fractional pawns of Vis came from an examination of the book economy of the Order of Hermes; its' beginnings are as a (pardon the pun) bookkeeping convention. It is not suggested that fractional pawns are usable for enchantment, or study, or spellcasting, or other magical pursuits; just that they are used to accurately account for the value of other things which are not Vis. No breakthrough is needed.
If someone wants to use the fractional pawns for magic instead of economics, then they will need a pile of them. Or, as you say, a single fraction & a Breakthrough based on Soqotan magic -- although I think the Soqotans use thirds of a pawn, and we are talking about fourths or fifths here.