Yes, it would seem that the "Desired Election" box with a $ sign in front of it will be the per pay period about you want contributed. I haven't seen it from an official source yet, but the projections I've seen are that the 401(k) salary deferral limit will be going up to $19k in 2019, so you would want to put in $730.76. Your wife's employer will most likely reduce the final contribution rather than let her go over the limit, so you don't need to be super careful about coming in just under the limit.