1. Our manual says field elevation, but frankly don't think it really makes any difference. (There is a special case when landing at airfields whose elevation is > 6000' see the Supplementary Procedures)
2. Assuming that you have already consulted your normal landing distances Vs Flap Vs brake settings, then my general rule of thumb is if my Vapp (i.e. Vref+additives) is going to be 150kts or more using Flap 30 then I use Flap 40. However if turbulence or windshear is forecast or is likely, or reported, then I stay with Flap 30 and wind the brakes up accordingly.
The reason why: I used to fly for an outfit that had the BTMU option. It seemed to me that we often parked with hot brakes, after landing Flap 30, Vref>150Kts, AB3 and/or "stomping" on the brakes to make turn offs. Whereas Flap 40, AB 2, Vref <150kts rarely resulted in hot brakes. Just my observations over the years!