My issue with both 170 and 171 is that they both make assumptions: 170 that the fork axis is along the steerer axis. (I have heard it said that Peugeots of the day often ere built with forks that "trailed" a touch, ie where the fork axis was steeper than the steerer axis.) The 171 assumes the top tube is horizontal. On traditional bikes, usually, yes. But on a hastily jigged bike. maybe not.

I would prefer tools that give correct measurements without assuming good workmanship.

Ben