When you remove the lens unit, turn it over and remove the metal case.
Then you will find what I call the Lens unit Mobo.
There is a pot in the middle of it (see picture)
My Viper GC didn't read a lot of brands 8 mm disk.
If your value is lower than 210 Ohms, Leave it alone :-)
In my experience 195 ohms work perfectly.
When to Tweak ??
I would tweak it if your value is greater than 250ohms (but if you already opened the cube, you could just easily tweak it and save yourself the annoying trouble of DVD-R media not booting.)
My value was 380 ohms. I reduced it to 250 and I have had no problem since. (almost every brand 8mm Disk I tested Emtec/Sony/optodisc/Memorex work perfectly Burned @ 2 speed)
I have "vipered 2 more GC's" and measured their Values
I was supprised to see that their standard values were between 180 and 250 OHMS.
So that's why I consider the Value optimized between 170 and 210.
Hires picture: 
Sony Habdycam 8cm DVDR 1,4 GB (burned @ 2 Speed)
Optodisc 8cm DVD-R 1,4 GB (burned @ 2 Speed)
Memorex 8cm DVD-R 1,4 GB (burned @ 2 Speed)
Lead-Data 8cm dvd-r 1,4 GB (burned @ 1 Speed)
Emtec DVD-r 1,4 GB (burned @ 2 Speed)
This counts for the US and PAL CUBE (haven't tested on the Panasonic Q (don't even know how to "viper" that one) and haven't tested it on the JAP cube)