The paved garden steps at the Casa de Cowboy have gaps under the steps where it wasn't possible to add paving, currently filled with weed-generating and ant-attracting loose aggregate.

I was thinking of replacing it with cement-stabilised aggregate to lock it in place and to deal with weeds.  I don't want to concrete it up, just deal with the problematic loose aggregate.  I've got a leftover bag of Drymix Quickset Concrete and a bag of builder's mix, the Drymix contains 10-30% cement (from the MSDS).  For cement-stabilised aggregate I'm looking at around 5% cement, assuming the 10-30% equates to roughly 20% cement (for a 1:4 ratio), mixing 3 units builders mix to 1 unit Drymix should give a 1:19 ratio or roughly 5% cement which seems to be what I'm after.

Does that sound about right?