First it should be `floor(NSFoundationVersionNumber) < NSFoundationVersionNumber_iOS_6_0` as we want less than iOS 6.0:

Also both 6.0 and 6.1 values are the same in `Foundation.framework/Headers/NSObjCRuntime.h`:

```obj-c
#if TARGET_OS_IPHONE
#define NSFoundationVersionNumber_iPhoneOS_2_0	678.24
#define NSFoundationVersionNumber_iPhoneOS_2_1  678.26
#define NSFoundationVersionNumber_iPhoneOS_2_2  678.29
#define NSFoundationVersionNumber_iPhoneOS_3_0  678.47
#define NSFoundationVersionNumber_iPhoneOS_3_1  678.51
#define NSFoundationVersionNumber_iPhoneOS_3_2  678.60
#define NSFoundationVersionNumber_iOS_4_0  751.32
#define NSFoundationVersionNumber_iOS_4_1  751.37
#define NSFoundationVersionNumber_iOS_4_2  751.49
#define NSFoundationVersionNumber_iOS_4_3  751.49
#define NSFoundationVersionNumber_iOS_5_0  881.00
#define NSFoundationVersionNumber_iOS_5_1  890.10
#define NSFoundationVersionNumber_iOS_6_0  993.00
#define NSFoundationVersionNumber_iOS_6_1  993.00
#endif
```