For ariyala's calculator you want to first select your class and race, then click and drag the ilvl sliders at the top to indicate the ilvl range you have available to you. After that just use the 'BiS Solver' which comes preset with that job's stat weightings and accuracy cap to help find the best options (just hit 'solve', you don't need to mess with the numbers).

In case you don't know what stat weightings are they're essentially how big an impact a given stat has on your performance and has been calculated by the community through a great deal of testing. In the case of my Summoner, using Intelligence as the baseline with a value of 1, each point of weapon damage is worth 6.9 times more than a point of INT whilst Spell Speed is only worth a measly 0.098 (it's useless for SMNs). Unfortunately there's no resource in game for knowing these values.

It's worth noting that the difference in effectiveness between multiple pieces of the same ilvl is marginal for most jobs. There is a difference, but unless you need to squeeze every last drop of efficiency out of your equipment you're normally fine just meeting the accuracy cap for each turn and then just getting your ilvl as high as you can. The only real exceptions are skill/spell speed on BRDs and SMNs as the stat is essentially useless to them and a complete waste (in fact in SMNs case it hurts them as it causes them to burn MP quicker through Ruin spam).