yeah, I think you are making this harder than it needs to be. If your night time temperatures are staying in the upper 40's or especially if they are staying in the 50's, then you aren't worried about cold. You are just giving your tomato plants a chance to get used to hot sun and wind a little gradually. You don't need to bring them back indoors at all.
I do hardening just by increasing the amount of exposure, by location not time. Since until recently I had to go to work all day, I couldn't be messing with moving things around every couple hours. So I just brought the plants out and put them in a location with no direct sun and left them there. Only brought back in if it would be too cold or windy for them (wind is very hard on tender seedlings). After a couple days of that, moved them to a location with just a few hours of morning sun and left them there. If they are ok after a few days of that, moved them to a location with a few more hours of sun. So weather conditions cooperating within a week to ten days I would move them to their final location, but let them sit there for a day or two still in the pot, before planting.