Itrip = Vref / (8xRs)
Rs is 0.1R, so with Vref at 0.6V, Itrip is 0.75A. This is reduced to 0.525A by the A4988 chip for full step operation. The motors are rated at 1.7A, which equates to a Vref of 1.36V (actually 1.9V when you allow for the full step current compensation by the A4988). So you have plenty of room to increase the Vref. The tradeoff with increasing Vref is the extra current means the A4988 will get hotter, not really a problem until you get up to say 1.5A. So my recommendation is to set the Vref to 1V and see how that works (I’ve set mine to 1V, a bit more on the X axes).
The only one you want to be careful of is the Z axis which is always on, so you’d want to increase that one slowly until it works satisfactorily. Unfortunately there is no way with the RAMPS board to wind back the holding current, which would be the preferred way of controlling the steppers when idle.
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