Are you sure the 705 allows you to manually enter a slope? I was under the impression you could only check it.
I don't have a 705 or wireless SRM so I've not done this. My impression was there is a buried feature in firmware 2.6 that allows one to enter SRM slope. According to the SRM FAQ on thier homepage:
Q: Can I manually enter the slope of my PowerMeter into a Garmin Edge 705?
Yes! Garmin has released an update to their firmware, connect your Garmin to your computer and update it through their website. Then, you'll be able to enter the slope by doing the following:
Go to Settings > Ant+Sport > Accessories
Move the thumbstick left > right > left from any field on the page, to bring up a new page
The top number on the screen will be the ID number of the PowerMeter that is connected
The field below it allows you to manually enter the ID of the PowerMeter you want to work with, or to enter "0" to get the Garmin to search for any available PowerMeter
The second enterable field is called "Slope", and here's where you can manually enter the slope of your PowerMeter, if you wish to do so.
Not being able to adjust the slope on an SRM is an obvious downside because these things do drift and the factory calibrations (slope) are notoriously wrong.