How can I tell if my font and outline selection is the right size?

I'm working on subtitles for a film, and the broadcaster has specific specs for subtitles. Namely, the letters should be 34 pixels high and the outline should be 3 pixels in size. The problem, of course, is that FCP doesnt give pixel size when doing font or outline size. 

I'm using Helvetica Bold. Is there any way I can try to calculate what font size I need to be 34 pixels high and what value I should give for the outline size so it is 3 pixels?

(Below I've attached exported still frames who's name indicates the font size value. In all of them, the outline is set to 1 although I've also added two with larger outline. I don't know if someone can figure it out just from this image)

This will depend on the frame size that you are delivering. 34 pixels high will look different on 4K or 1080p.

My advice would be first to ask if you can be given an example image of what the end result should look like. You could then bring that image into Final Cut Pro to line up with what you are doing.

Assuming your project is the same resolution as the final delivery, you could export a single frame of your project as a TIFF and open it in Photoshop. Then in Photoshop/Settings/Units and Rulers set the Type units to Pixels. Create your own example of 34 pixel high Helvetica text in Photoshop. Outline this by 3 pixels in Photoshop. Then bring this image back into Final Cut Pro to use as a guide.

Another thing to add that might be useful. In Final Cut Pro settings/General/ you can set the inspector info units to pixels. This won't allow you to set the fonts in pixels but it means you can position things across the screen in pixels. So, if you use a shape on screen as a guide, you could copy that shape and move the copy 34 pixels higher on screen, giving you a kind of guide to what 34 pixels looks like in your current frame.

But as I said at the start, the first thing you need to pin down is what video resolution this 34 pixels spec refers to.

Below I've attached exported still frames

Your images aren't showing here.