Again a time question...HH:MM:SS or not

it has got nothing to do with showing seconds or not..
the problem is that your formula is wrong. a day has only 24 hours by using the HOUR() formula it will never go over 24 hours
the hour formula cannot give you duration. it gives you only the hour of day, any value over 24 hours is assumed to be the next day so 25 hours is presented as 1 AM

to convert a duration time to decimal all you need to do is C6*24 then you will have the excact decimal value of any time, eigther under or over 24 hours duration

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additionally: to present duration in a time format you need to set the cel formatting from hh:mm:ss to [hh]:mm:ss
with this last format it will show duration times over 24 hours correctly you find the setting under cell properties > custom


Edit: added example file with separate explain tab for converting and formatting time and date

Try with

Formula:

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Format to Number