If you can get a 6dB or 10dB pad you will probably be much happier, since you will have a degree of safety margin.
OR, you can go the cheap-easy-sleazy approach: if you have an oil-filled dummy load (e.g. the MFJ), you can put a BNC connector into it, and put a small probe loop inside. That will couple to the RF, and give you an uncalibrated, but attenuated signal out.
Then, you can perform a calibration at the frequency of interest: take your HT, put it on that frequency, feed it directly into the service monitor, and take a power reading. Then, feed it into the dummy load, and connect your newly added probe port to the service monitor, and measure the level that way. Subtract one from the other (in dBm, of course) and you get the attenuation of the can. You can now run the measurements.
I did that when I was working on a 300W tube PA with my COM-120B.
You can also do something like that if you have an isotee (e.g. http://urgentcomm.com/test-and-measurem ... -one-these
, or if you have a Pomona connector kit (http://www.mouser.com/ProductDetail/Pom ... 5LgHWzRSbj
)), you can use the isotee and a dummy load to do the same thing.
(if you have a 1900, you REALLY should get the Pomona kit - best $370 you will ever spend.)
This is my opinion, not Aeroflex's.
I WILL NOT give you proprietary information. I make too much money to jeopardize my job.
I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.