You can probably mix this with water to make your own "Prime" solution. Since you pick the amount of Safe and water, you can make the measurement whatever is convenient for you. I assume it will be shelf-stable mixed like that, Seachem's customer support is helpful and can tell you for sure.
Or you can just break out the good old fashioned mirror and razor blade, and split up a 1/4 tsp. into five equal portions for a 10G.
No worries if they're a little off.
I know it involves a purchase, but another option is a set of measuring spoons:
I think I recall seeing these cheap at Walmart, and the pinch spoon, at 1/16 tsp, is perfect for a 10G if you underfill it just a little.