You could add a few bits to the schematic, such as abs(v(c1)) with an ideal diode + capacitor, then use that voltage, say v(max), to divide the final answer, v(c1)/v(max). If you want to avoid doing this manually, everytime you plot v(c1), you could simply add a B-source to your liking with that expression.
There could be another solution with the integrated S&H, but that would require a clock, or others, it's just a suggestion.
Vlad ______________________
-- holding, among others: a universal analog/digital filter, block-level models for power electronics (and not only), math blocks
with a more stream-lined approach, some digital ADC, DAC, (synchronous-)counter, JKflop, etc.