Cvt fluid change

I'm sure this has been asked before but searching around I didn't find much. 

We have switched our verso from diesel to petrol automatic. It's 2010 reg so I believe fitted with k311 gearbox. It's done 93k miles and I'm keen on servicing CVT gearbox as it  sometimes hangs on to lower gears for bit too long on flat road and paddle shifter fails to force a gear up too. I'm thinking it'll be cheaper to service it now then replace it later, if it gives up on me. Hopefully servicing will prolong the life of gearbox.

I've seen the method of refill and drain couple of times but really don't like that. I'm more towards taking the sump pan off clean the magnets and change the filter.

Has anyone here done it that way? Anything I need to be aware of other than measuring the fluid and putting back exact amount?

Also would 5l be enough? According to manual it takes over 7 ltrs for dry gearbox but you can't drain torque converter and some other areas completely. 

Anybody able to drop me a technical manual if they have access to Toyota official document?

It seems quite an expensive fluid too. I've been quoted £120 by local Toyota dealer for 5L. If anybody knows a cheaper place please let me know. Fluid must be either by Aisin or Toyota as I don't like mixing 2 different makes or putting some third party fluid in. It's too critical of a part.