how to calculate square tapered BB spindle length when coming from external threaded bearings

The spindle length is defined from the crankset, not the frame. There is no relationship between your Tiagra (external) and the future FSA (tapered). You should find with the FSA crankset the data of the required spindle length.

First of all, that crank is one of the most problematic bike parts I've ever touched. They bend and fold their weirdo made-up-BCD chainrings with amazing regularity. I've had to deal with them a lot. They're bad.

To answer your question: most cranks have technical specs available to tell you the recommended spindle length as well as the chainline. 4600 implies you're presumably putting it on a non-disc road bike and would ideally have a narrower front chainline providing your frame gives you the clearance. So first you're going to measure or look up your rear chainline, then you're going to confirm however you want that the frame will give you the clearance you need in front if you set up the 46/30 for optimal chainline. Then, you're going to subtract whatever the chainline you want is from the published chainline, divide by half, and get a spindle that much shorter. (Or longer, if you were widening the chainline).