I calculate core CY value everyday based on the following formula:
CY - SPWR =(((B3*142.4)+476)-(52.033*P3))/142.4
B3 = current CY price. 142.4 millions shares outstanding. net debt 476 millions = (CY's current asset - CY debt) - (SPWR's current asset - debt) P3 = SPWR current price CY holds about 52.033 millions share.
Based on today's closing price, CY - SPWR = 5.06 - all time low.
As to your valuations, yes that is one way of doing it. I think it's interesting though that if you subtract CY's stake in SPWR from CY's market cap, you are left with a measley $130 million based on today's closing prices.
If CY stays at this level and SPWR pops $2.5 (yes exactly 2.5), the market value of 52+ million SPWR shares will slightly exceed CY's market cap.