Sodium lactate is an ingredient I can't do without. The tiniest amount (I get by with 2%) makes such a difference in the fluidity and hardness of hot process soap. Having a fluid HP makes it much easier to get into the molds. A quick hardening makes it easier to get back out.
Sodium lactate is also a very forgiving ingredient. I've added it at just about every possible stage of the soap making process and it works every time. I've also used it in both the powder and liquid forms, again with consistent success. The only time I've ever had a problem with sodium lactate is when I've added too much. Anything at or above 5% makes my soap crumble and break.
For best results, I prefer to make my own solution. To make this solution, I weigh out sodium lactate powder equal to 2% of my batch weight (usually 50g) and then multiply the amount by 5 to figure out how much water I'll need (250g). I subtract the water from my total recipe, mix it with the powder and add it back to the soap at a light-to-medium trace. A stick blender is very handy for getting it to incorporate. I always get soap that will easily sieve through a slotted spoon and then harden up to candle-wax consistency within a few hours.
If you enjoy making hot process soap, I can't recommend sodium lactate enough. Even a small amount in a weak solution turns out a great batch of soap.