1) When SubSeeding, it's usually a better idea to use Substitute first, so that you can ensure that you can set up Leech Seed right after. Usually, setting a Substitute is harder than using Leech Seed, as if your opponent attacks you have to face the risk that it will break. But depending on the situation, it's possible to Leech Seed first and then set up a Substitute.
2) Substitute has 1/4th of the user's HP. Say you have a Pokemon with 160 HP, and it uses Substitute. The Substitute will have exactly 40 HP. When the original HP is indivisible by 4, it will take the lower alternative. Like if the Pokemon's HP was 161 instead, the Substitute would still have 40 HP. This allows you to set up 4 Substitutes without any healing, unlike before.
Hope I helped!
