Hello Arara1kins and welcome to the forum.
Arara1kins wrote:the gain of the interface to the guitar input should be inserted so that the gain peaks at the input to the track in the daw were up to -6 dB.
This is sensible advice, as long as -6dBFS is the absolute loudest peak. Much closer to 0dBFS and you're getting a bit too close to clipping for comfort. And you must remember to view this on your DAW's meters, before you insert any effects.
Arara1kins wrote:in the amplitube 5 the same rules for calibrating the input gain of the ADC?
Once you've got the gain set on your interface you're all good. The Input level control in the bottom lefthand of the AT5 GUI is software based and will not affect the dry recorded guitar if you turn it up as the A-D conversion has already happened in the audio interface. It is best practice to not let the input level shown in AT5 go into the red, but if you need it to drive your tone and it sounds good to you then I say go for it. Where would be if the Beatles hadn't overdriven the desk input for Revolution 1 on the White Album..
If you realise you don't like it further on down the line you can just back off the Input level in the GUI as your recorded dry guitar will be whatever it was when you set the gain on your interface, something the Beatles wouldn't have been able to do
Hope that helps.