I mainly use those calculators to calculate the significance if the test is relevant: http://getdatadriven.com/ab-significance-test https://vwo.com/ab-split-test-significance-calculator/ Would be nice to see a custom one made by Clicktrait anyway
If you have got a page with low traffic then I would suggest doing the innovative A/B tests. That means, that you change more stuff at once but you still do not do the whole radical redesign. Innovative tests are perfect for small sites because doing A/B test testing out each element separated would take years. Problem is that you find the final result but you don't know if any of those elements actually lowered the CR while another one increased it A LOT.