When you find the end of your field lines and start backtracking to find septic tank to see if full about how long of footege?

Every system is going to be different based on location and soil type. If I were trying to find the septic tank, I would draw a line straight out from the house closest to the bathroom and another at 90 degrees stoward the lateral lines. Where the lines cross should be close. Depending on how deep the tank is, there may be darker grass or a dead spot depending on weather, rainfall or the lack of.