Eyepiece /early improvement recommendations

Hi all , I’ve recently got a 70mm diameter by 400mm focal length refractor. I have a 24mm and 6mm eyepiece (the 6mm I’m sure is useless)

Sorry I did check Google , and chat got first but it’s not very human 🤣

What is the highest zoom eyepiece people have got away with using ? In similar set ups.

I have googled the equation for working it out and an a little bit stumped.

Seems the answers to my current magnification would be
400mm ÷ 24mm = 17x zoom
And
400mm÷6mm = 67x zoom

Although I’m not sure what the max zoom capabilities of the scope are or how to work it out.
Any help with learning this area of workings would be greatly appreciated, as I’m looking to expand my eyepiece collection, but learn what I need first.

Thanks!