This is what Pete Hamill said in his review of the new book on Mays. See Willie Mays, the Say Hey Kid. Here is what I came up with on this.
I find it hard to believe that Mays lost 100 HRs because of Candlestick. Using the Retrosheet data, I found that from 1960-71, his home HR% was 6.72%. Road, it was 5.80%. He only played part of 1972 with the Giants. His home % was 15.8% better than the road % (since 6.72/5.80 = 1.158)
For the entire NL in this period, the home HR% was 2.34% while on the road it was 2.25%. The home % is 4.2% higher than the road % (since 2.34/2.25 = 1.042).
Mays seems to have gotten more out of his home park that most players. If we give him 100 more HRs at home, his home HR% rises to 10.04%. That is about 50% better than what it really was and about 73% higher than on the road.