I need advice for level design.

hi all.

im working on a runner prototype. i finished to elements for my levels. (health, mana, speed boost, damage giving stuff, mana giving stuff, shield) now im putting them on level but little bit confused about what should i do first. should i put them where i want and than adjust values (like “+25 mana” item, “+30 health” item) or decide values first and than work on places to put them. or maybe that 2 way should go synchronized? i realized its getting harder to balance after add more elements.

could you suggest me a book, website, trick to make it more convenient, more balanced? i read “ultimate level design guide” but its not much focused on balancing.

(sorry for bad english)