I've found template for calculating confidence for A/B tests. I need it to calculate confidence for custom metrics. At first I tried to verify if it works correct. I created regular A/B test and compare the results on 'report'-tab in target with the results from that excel-file.
1. If conversion rate is small ( less than 1%), then results are identical
2.If conversion rate is not so small ( greater than 2%), then results are not the same. Confidence in template is always greater.