That’s too much detail, the 6th decimal would indicate a precision of tens of centimeters, which your GPS probably does not really give you, and your use case probably does not need either. Just 4 decimals should suffice; see Best practices when sending GPS location data [HowTo]. That also describes how to send and decode negative values.
Please also go back to my reply to your earlier question; What is the maximum number of characters I can send with LMiC? - #6 by arjanvanb You should really understand the two examples I gave there, before even trying to encode and decode data. If not: follow the links to the converters in that answer and try some values yourself, and take time to re-read Working with Bytes.