Most of you expert traders and academicians have better tools and test environments and literature than me.
So, can any of you experts confirm my finding as described in
this posting along with an example data set?
Just for the historical record: here are the Q&D crypto-like

results of my test program.
Background story: I added calendar spreads just today to my "vertical put spread" program,
and promptly made the said observation and discovery.
But, as said, everything is very new / very fresh, so I just hope it's bug-free.
As a control instance I'm calling you experts out there to test/verify/confirm this finding.
Below, I'm focusing on these 2 outputs (PnL% for Spot and IV same at expiration of the ShortPut):
Sx_at_SP_S=100.0000(dPct=0.00 PL=5.8656 PLpct=6.23)
Sx_at_SP_S=100.0000(dPct=0.00 PL=12.2449 PLpct=12.37)
As can be seen: PnL 6.23% vs. 12.37% is of course a very significant result... Q.E.D. IMHO
Code:
PgmArgs(16): ./yfapi_get_options_data.exe CSP+LP_calc 1 3 0 100 100 60 100 -1 150 100 60 90 -1 150 >$FN
fErr=0 mfac=1.0000 SP(S=100.0000 DTE=60.00 K=100.0000 Pr=23.8934 IV=150.0000) LP(S=100.0000 DTE=60.00 K=90.0000 Pr=18.0278 IV=150.0000 --> Pr_at_SP_expiry=0.0000 Net_Pr=18.0278 Used_Pr=18.0278 Pr_for_SP_DTE=18.0278 Saved_LP_Pr=0.0000(0.00%)) :
SM=0 CashAcct/3 PutVerticalBullSpread NetPr=5.8656(Credit) CashReq=10.0000 CBraw=94.1344 CB=94.1344 NetAssReq=76.1066 MaxPL=5.8656(6.23% @Sx>=SP.K(100.00)) MinPL=-4.1344(-4.39% @Sx<=LP.K(90.00)) WLrat=1.42 LWrat=0.70 WIrat=16.05 LIrat=-22.77 BE(Sx=94.1344(-5.87%)) S0(PL=5.8656(6.23% Mon=3.11% Ann=44.44%) Irat=16.05) SL_Score=168.77
Sx_at_0=0.0000(dPct=-100.00 PL=-4.1344 PLpct=-4.39)
Sx_at_LP_K=90.0000(dPct=-10.00 PL=-4.1344 PLpct=-4.39)
Sx_at_(LP_K+Sx_at_BE)/2=92.0672(dPct=-7.93 PL=-2.0672 PLpct=-2.20)
Sx_at_BE=94.1344(dPct=-5.87 PL=0.0000 PLpct=0.00)
(Sx_at_BE+SP_K)/2=97.0672(dPct=-2.93 PL=2.9328 PLpct=3.12)
Sx_at_m1SD=86.5731(dPct=-13.43 PL=-4.1344 PLpct=-4.39)
Sx_at_SP_S=100.0000(dPct=0.00 PL=5.8656 PLpct=6.23)
Sx_at_p1SD=115.5093(dPct=15.51 PL=5.8656 PLpct=6.23)
Sx_for_PLpct_50=100.0000(dPct=0.00 PL=5.8656 PLpct=6.23)
Sx_at_SP_K=100.0000(dPct=0.00 PL=5.8656 PLpct=6.23)
Sx_at_SP_K*1.25=125.0000(dPct=25.00 PL=5.8656 PLpct=6.23)
PgmArgs(16): ./yfapi_get_options_data.exe CSP+LP_calc 1 3 0 100 100 60 100 -1 150 100 90 90 -1 150 >$FN
fErr=0 mfac=1.0000 SP(S=100.0000 DTE=60.00 K=100.0000 Pr=23.8934 IV=150.0000) LP(S=100.0000 DTE=90.00 K=90.0000 Pr=22.8528 IV=150.0000 --> Pr_at_SP_expiry=11.6485 Net_Pr=11.6485 Used_Pr=11.2043 Pr_for_SP_DTE=18.0278 Saved_LP_Pr=6.8235(37.85%)) :
SM=0 CashAcct/3 PutVerticalBullSpread NetPr=12.2449(Credit) CashReq=10.0000 CBraw=98.9594 CB=98.9594 NetAssReq=76.1066 MaxPL=12.2449(12.37% @Sx>=SP.K(100.00)) MinPL=2.2449(2.27% @Sx<=LP.K(90.00)) WLrat=5.45 LWrat=0.18 WIrat=8.08 LIrat=44.08 BE(Sx=0.0000(-100.00%)) S0(PL=12.2449(12.37% Mon=6.09% Ann=103.33%) Irat=8.08) SL_Score=209.01
Sx_at_0=0.0000(dPct=-100.00 PL=2.2449 PLpct=2.27)
Sx_at_LP_K=90.0000(dPct=-10.00 PL=2.2449 PLpct=2.27)
Sx_at_(LP_K+Sx_at_BE)/2=45.0000(dPct=-55.00 PL=2.2449 PLpct=2.27)
Sx_at_BE=0.0000(dPct=-100.00 PL=2.2449 PLpct=2.27)
(Sx_at_BE+SP_K)/2=50.0000(dPct=-50.00 PL=2.2449 PLpct=2.27)
Sx_at_m1SD=86.5731(dPct=-13.43 PL=2.2449 PLpct=2.27)
Sx_at_SP_S=100.0000(dPct=0.00 PL=12.2449 PLpct=12.37)
Sx_at_p1SD=115.5093(dPct=15.51 PL=12.2449 PLpct=12.37)
Sx_for_PLpct_50=100.0000(dPct=0.00 PL=12.2449 PLpct=12.37)
Sx_at_SP_K=100.0000(dPct=0.00 PL=12.2449 PLpct=12.37)
Sx_at_SP_K*1.25=125.0000(dPct=25.00 PL=12.2449 PLpct=12.37)