Your existing maths is actually rounding down due to the 'subtract 0.5', which will result in a darkening of the image (since everything rounds down to the next "step"). By re-arranging the nodes and using an Add at the end of the chain instead of in the middle, the logic will round to the nearest "step" and the final 'Add' will allow you to adjust the final 'offset' to get the desired result (you can also adjust the Multiply and Divide factors to scale the output).
Here are the adjusted nodes (note that I've added Value nodes to allow easier adjustment of all channels simultaneously) :
And here's the result :
Note also that I've adjusted the Multiply and Divide factors from your example of 7 and 6 to produce similar results since I didn't know your light settings for your original image (so had different starting levels).
Your original example split the image into 7 steps (due to the multiply by 7 before the 'round'), rounded down to the next lower step (with the subtract) then multiplied by 6 (so the result is actually lower than the original). In my adjusted nodes, the Add can be adjusted to regain some of the loss, or the Divide adjusted to change how the steps are converted back into the final range.