I hope if you look at it, it will make sense to you, and hopefully you can adjust it to fit your needs. At the same time, hopefully this will give you a better understanding of payloads.
I would like to mention one thing. You asked about being about to say the payload optionally. There is a way to do this.
You can create two separate commands. The first could be the simpler command.
"make lights brighter" >>> and this command might turn the brightness up by 10%
another command could use the same wording but with an extra payload. So you could create a command like this:
"make lights brighter by (10,20,30,40,50) percent" >>> here I am implying that the command is using a payload list with the (10,..,50)
usually the speech engine will be smart enough to wait a bit after you say "make lights brighter" to see if you will also say "by x percent". But in some cases it may not... it may just assume 10% immediately, so if you really want to make sure you get the right result you should use a different phrase completely for the second command. Try it though and see what happens!