Selling Price When Only Costs and Gross Margin Are Known

I have many situations where I know the cost of goods and the gross margin wanted to be achieved. Please remember, this is gross margin, not net and I don't know the net margin, I only know the costs and that I want to achieve a certain gross margin. So I've got it this far:

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So for a project which has a cost of $100 I need to sell the product for $138.89 to achieve a 28% gross margin. And all I have to do is enter the cost and the gross margin I want. The formula in cell A2 seems to work pretty good but if I place a gross margin of say 125% in cell B1, I get some not so accurate results in cell A2. I gotta believe there's a way to do this so we can quickly determine the selling price to achieve a given gross margin knowing only the margin and the costs which will work for any number placed in the gross margin cell.

Thanks quekbc. When I think about it that way it's clear. I guess that's why the sales price goes astronomical when I put in a 99.999% gross margin.