Yeah, measuring existing suspension geometry is tricky and you can't really do it accurately enough to realistically predict steer effects, because you need the exact location of every pivot center. When you're adjusting bumpsteer, changing the height of the tie-rod end by very small amounts affects your steer response, I mean like .050" or less is enough. That kind of measurement accuracy is not really possible with ball joints & tie-rod pivots since they're encased in their housings. For that exact info you have to either section an existing part to measure it, or have an engineering drawing for it.
You can measure all the geometry to construct a model and see how things generally work, it just won't be a predictive tool for bumpsteer.
McStrut geometry:
Basically the same as dual A-arms, but the UCA is replaced with the strut. The terminology in this pic is a little different; "Virtual reaction point" is the instant center, "Pivot radius" is scrub radius, etc. Also the tie-rod and LCA are co-axial in this front view which makes them hard to distinguish. But basically you draw a line from the upper strut bearing, perpendicular to the strut axis; the intersection of that line with the LCA axis is the instant center.