How to calculate germination rate when testing seed viability

So I did find the tenth seed and it was sprouted too! Obviously this would work even better with a higher number of seed sprouted, but to get the basic percentage, sprout ten and you will know the approximate percentage. Why is it important to know your germination rate before seed starting? When if you are calculating the food you need to produce in the garden, then it is important to sow based on germination rate not number of seeds. 75% germination rate would require sowing an extra 25% to ensure an adequate harvest.