Yep, secondcor521 has it right.
The number of dollars you get to help pay your premium is a fixed amount based on your MAGI, family size, and cost of the second-cheapest silver plan in your area. You can use those dollars to reduce the cost of any plan on the exchange, regardless of metal level.
Do be aware that if you can bring your MAGI below 250% of the federal poverty level, you will likely qualify for cost sharing subsidies in addition to the premium subsidies. These cost sharing subsidies are only applicable to silver plans, but they have the effect of making the silver plan's coverage better than normal. Between 200-250% of the poverty level the effect isn't huge, but below 200% you effectively get a platinum plan for the price of a silver plan.