Applying 18-6-12 fertilizer

My lawn is about 3/4 acre in total. Grass growth has been weak in many parts and a lot of moss has appeared throughout large parts of the lawn, particularly since last summer. I applied sulphate of iron at about 7.5g per sqm last week and will be scarifying the worst affected mossy areas this weekend to clear out the black moss. I was then intending applying some fertilzer and overseeding areas with poor grass growth. With the size of area to cover, I have just bought some general 18-6-12 fertilizer from a farm co-op. At what rate should I be applying the fertilizer and how long afterwards should I wait to spread new grass seed ?