Photoshop :: Accurately Zooming

Can I zoom more precisely than just by clicking with the zoom tool?

I have photographs of a medical patient. I want to accurately measure certain medical features, to see if they are getting larger or smaller over time. I figure that I can use guide lines to measure - but that would require me to have each photo starting out as the same scale, and that's why I need to be able to accurately zoom.

I suppose I could resize, but that would seem to take a very large amount of trail and error. If I can zoom arbitrarily, I believe I can use layers with top-layer alpha = 50% or so, and match any two photos to each other.

Or is there a better way altogether? It's not feasible for me to have each photo be natively as the same size/distance as every other, so I have to adjust the photos themselves.