10 is probably a safe bet, but it is all dependent on what the current coverage and overlap look like. Also, the wireless client may still try to go back to the poor AP, depending on the algorithm used in the driver. I would use it more as a last resort. More information on Enhanced Roaming here.
What I would suggest is taking a look at the SNR reported by the client at the point that you think it should have roamed to the other AP. You can use the wireless adapter on the device, or cnMaestro, or a free Wi-Fi utility. That will be a good indicator of threshold.
However, this may cause disruption of other clients, so you may need a larger sampling of wireless clients to ensure that the value being used isn't going to adversely affect your design in general. Take a look at the attached, but if you have a smallish number of laptops, I would see if you can change the adapter settings first.