The recumbent bike in my avatar has has a similar front suspension and it works great. Because there is no sliding action the friction is very low which helps the suspension respond to bumps.
I also wanted to use a spring that I had.I made those 2 red springs by cutting one long spring in half and closing and grinding the cut ends. Each spring has 13.5 coils, a wire diameter of .170" and a coil diameter of .970". From that I can calculate that each spring has a rate of 97 lbs per inch. So it takes 97 pounds to compress the spring one inch. I also designed the fork so the ratio of the leverage from the wheel to the pivot point versus the leverage from the spring to the pivot point is 1.3:1. From this I can calculate that it takes about 115 pounds to displace the wheel one inch. I estimate that the static weight on the front wheel is about 90 pounds. That's the condensed story, I actually built up a spread sheet to try many combinations.
This might be more info than you wanted, but for me that's part of what makes building a bike interesting