setting conservative does a bit more than just cahnging reltol. from what i remember it changes stuff like ensure high order is on (which makes the noise floor much lower), and changes other setting like lteratio, steady ratio and limits the timestep size etc. Anyway the accuracy/tolerable convergence error is a function of reltol and other things like vabstol, lteratio etc. For definitive detail, just look it up in the spectreRF reference manual.