normal Standard conditions
Define "normal standard conditions". What I would suggest is that you show your captain the following excerpt from the FCTM - granted, it's not a precise value, but it comes from Boeing itself.
Idle thrust is adequate for taxiing under most conditions. A slightly higher thrust setting is required to begin taxiing.
An instructor I've flown with uses half of the take off as a guideline maximum limit without obtaining a clearence from the tower.
Half of what? N1 is not a linear indication and 50% N1 will not give you one half of thrust at 100% N2.