I'd buy another 1/2 gal (or 2qts), gang mix all of it together before you add anything and plan on keeping the 2qts for future use/match needs. At 5qts ready to spray you won't have much margin for error, you'll also probably blow right through it if you plan to cut and polish the finish. If you're good with the gun and have a clean area you might like it as-is when sprayed out. My worry would be coverage if it's ready to spray with the addition of the catalyst. I'd wonder how "thin" it really is and then will you have total coverage over a grey base (yes, even a light grey base). Does part of your plan include sealing it 1st? If so you want a white sealer. Even light grey gets "dark spots" when mil thickness is marginal, and since that's out there now too you want a light handy to fix your vision as you proceed. While not as bad as yellow you'll probably have your eyes get "tricked" into thinking it's light in spots (yellow can leave 'green' spots in your visual perception). Having a drop light handy will solve it where you might see dark spots that really are not there. Bright colors are a bit challenging that way. If you do plan to cut and polish you want a minimum of 3 coats, maybe a 4th reduced at least 20-25%, only the last one, not all 4 coats. Apply that last one with twice the flash time from 3 to 4 and being reduced it will head right for the floor if you get too heavy. Good luck, hope those ideas help a little.