Since it gets little use and you can keep it covered (can you?), maybe you can dose up to 20 ppm FC per the chart for 50 ppm CYA and see how long the FC lasts. If it doesn't last long enough you could try raising you CYA to 80 ppm (per xpert and mas' suggestion) and then dose to 31 ppm FC. If you dare you might be able to to go higher but remember that prolonged use could damage heaters, vinyl liners (what is the surface?), etc. Swimming above shock FC levels is not recommended so you would have to time your dosing with the swim usage (high FC levels can also bleach hair and swimsuits). Does not sound easy to me...