While calculating for K+ where are the cursors supposed to kept at: at the stable traces or near the rise side?
Since K currents is (usually) not inactivating, you probably want to measure it at a steady state (although for some K channel types you may want to measure at the peak). Then you need to divide the value of the current by the cell's capacitance, which could be roughly determined from the amplifier's dial or (better) measured with the single voltage step; the details are described in:
Platzer, D., Zorn-Pauly, K., 2016. Letter to the editor: Accurate cell capacitance determination from a single voltage step: a reminder to avoid unnecessary pitfalls. Am. J. Physiol.- Heart Circ. Physiol. 311, H1072–H1073. https://doi.org/10.1152/ajpheart.00503.2016