Building regs state that a gap of no more than 100mm is allowed.Just cut a piece of timber to 100mm (or less) and place it between your spindles.Fix the spindle in place and then repeat as you go along.On the stringers use the spacer and a spirit level.
Only problem with DAZB's method is when you get to the end you could be left with a 120mm space or anything else for that matter, theres not much chance of getting to the end and having a 100mm gap to match the rest.
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