We can base an approximate dose on what is known about SN, as the toxic dose of nitrite itself is the relevant factor not the sodium or potassium ion. The LD50 for SN is roughly 0.18–0.3 g/kg, and the molar weight difference between SN (approx. 69 g/mol) and KN (approx. 85 g/mol) should be accounted for if converting. This means you would need slightly more KN than SN to achieve the same effect, because the potassium ion adds more mass per mole.
At 150 lbs/68 kg, the commonly used SN dose of 25g in 50mL water is based on this range. To match that nitrite dose using KN, you would be looking at roughly 30g–33g of KN, but this is still speculative due to lack of real world data.
Also be aware KN is less soluble in water than SN, so it may take longer to dissolve or may not fully dissolve if not stirred well. There is less anecdotal safety/efficacy data, meaning you're in more uncertain territory. The same SN protocol still applies: antiemetics, fasting, calm environment, quiet timing, etc.
Because KN is rarely discussed, please proceed carefully and know that lack of documented results doesn't mean it's safer or easier.