I don't know about adding lighter fluid. Because lighter fluid is pertroleum based, and Future is water based, I wouldn't expect anything good.
The easy way to measure when dealing with a goop like X-21 is to mark a mixing bottle with the levels corresponding to the desired ratio. For example, if you're using a 4:1 ratio, fill a bottle with four parts of water, then mark the level. Now add another part of water and mark that level. Whatever you choose as a "part", be it a teaspoon, tablespoon, or someother unit would be dependant on the mixing bottle you use. I like to mix in large batches, so I use a larger mixing bottle.
After you've marked the bottle, fill with Future up to the first mark that corresponds to 4 parts, then add the X-21 until the Future rises to the second mark. Once it's there, you know that you have added a part of X-21 to your four parts Future even though you never separately measured as such.
Andy