Well, you can always start low and work your way up (regarding input power), starting with too much input can cause immediate damage.
With any unknown box that is RF sense keyed, I'll use a radio that has convenient carrier adjust and start at about half a watt and slowly increase the dead key.
A lot of people will disagree with i'm about to explain next..
I've heard people say as a loose rule of thumb "use just enough carrier to key the amplifier without observing any relay chatter".
The problem you can run into with too little carrier, is that if the radio is "peaked" or modified for "swing", the amplifier may produce a lot of distortion (over-modulation).
I turn up the dead carrier and use an occasional "Audiooo" to see where the amplifier starts to back swing with audio (on AVG reading meter), that tells me I am over driving the device(s). I will use this method to find a reasonable "maximum carrier output" number, and use that as my goal to reach (P.E.P.) while using a carrier (giver-or-take) 1/4 of the maximum dead key output, this leaves head room and hopefully will stop you from having a problem over-driving, flat-topping or sounding bunched-up/pinched.
If It keys 25 watts and "swings" 50 AVG, (so roughly 100 P.E.P.,) that is good!
You don't want to see any back-swing with an average reading meter, and you would ideally see the needle kick-forward (about 1.5X times the carrier).
You wouldn't want to have the amplifier produce an overly large carrier, this would sacrifice the amount of headroom or "swing potential" of the box,and your audio will sound "off" with too much carrier.
You can run the #'s on the power output devices and find a datasheet for each part (MRF455 or 2SC2879, or w/e is used) to see the manufacturers rated, maximum input specification.
It is hard to really see what kind of input to use without observing the output on a 'scope... That's one way to get a better idea of what is really going on, an oscilloscope will tell you more than a watt meter.