Need colour mixing advice please

[aclassiccase]

For various reasons I have a few different tubes lying around.
My hair is currently around an 8.40ish, maybe slightly darker.

I have a 6.40, a 9.0, an 8.0 and a 7.43.
Thinking I could mix the 9.0 and the 6.40 to make a 7.40?
Please help!

 

[AcidPerm]

What colour house are you using and what are you trying to achieve? Are you wanting to match mid lengths to roots?
Remember as you go higher, the colour pigment is less dense.