Your 4%/1000 ft is a crude approximation. Here is a table of pressure variations with altitude.
The 4% number comes from the ratio of the pressure at sea level to the pressure at altitude. So, at 5000 ft, for example, the ratio is 1/0.832 = 1.202, or 20.2% so if you fit a straight line to that you'd get your 4%/1000. But it works best for 5000 ft, i.e. your Santa Rosa site! If you work it out, the actual percentage "correction" varies from 3.7 to 5.8 in the table above. For altitudes less than 5000 ft, using your 4% number is conservative (i.e., safe); but for altitudes above 5000 ft it is not safe. Use table 9.4 in v7 of the navy dive manual. Here is part of it:
