The ganzhi years are extrapolated from the modern ganzhi date. This is something that I am not entirely sure is applicable, however, this is the method by which Ray Huang calculated his date for 1587, A Year of No Significance. The 23rd year of Chenghua was the final year with this reign title. The year 1488 begins with the use of the Hongji reign title. The campaign begins in 1501, which corresponds to the 14th year of Hongji, and the year of the metal rooster.
 
Age Trunk Branch Year Reign Year
27 yi  wei  1475 Chenghua year 11
26 bing shen 1476 Chenghua year 12
25 ding you 1477 Chenghua year 13
24 wu xu 1478 Chenghua year 14
23 ji hai 1479 Chenghua year 15
22 geng zi 1480 Chenghua year 16
21 xin chou 1481 Chenghua year 17
20 ren yin 1482 Chenghua year 18
19 gui mao 1483 Chenghua year 19
18 jia chen 1484 Chenghua year 20
17 yi si 1485 Chenghua year 21
16 bing wu 1486 Chenghua year 22
15 ding wei 1487 Chenghua year 23

Dates up to 1501
 
1506 bing yin (fire tiger) Wuzong year 1
1501 xin you (metal rooster) Hongji year 14
1500 geng shen (metal monkey) Hongji year 13
1499 ji wei (earth ram) Hongji year 12
1498 wu wu (earth horse) Hongji year 11
1497 ding si (fire snake) Hongji year 10
1496 bing chen (fire dragon) Hongji year 9
1495 yi mao (wood hare) Hongji year 8
1494 jia yin (wood tiger) Hongji year 7
1493 gui chou (water ox) Hongji year 6
1492 ren zi (water rat) Hongji year 5
1491 xin hai (metal boar) Hongji year 4
1490 geng xu (metal dog) Hongji year 3
1489 ji you (earth rooster) Hongji year 2
1488 wu shen (earth monkey) Hongji year 1
1487 ding wei (fire ram) Chenghua year 23
1486 bing wu (fire horse) Chenghua year 22
1485 yi si (wood snake) Chenghua year 21
1484 jia chen (wood dragon) Chenghua year 20
1483 gui mao (water hare) Chenghua year 19
1482 ren yin (water tiger) Chenghua year 18
1481 xin chou (metal ox) Chenghua year 17
1480 geng zi (metal rat) Chenghua year 16
1479 ji hai (earth boar) Chenghua year 15
1478 wu xu (earth dog) Chenghua year 14
1477 ding you (fire rooster) Chenghua year 13
1476 bing shen (fire monkey) Chenghua year 12
1475 yi wei (wood ram) Chenghua year 11

 Determining trunk date
 
0-1 metal geng/xin
2-3 water ren/gui
4-5 wood jia/yi
6-7 fire bing/ding
8-9 earth wu/ji