How are we to know if 12000458 is not supposed to be 12000458 ( a whole
number), or
120004.58 (2 decimals) or even more decimals?
Is there some criteria?
Or is the string number value ALWAYS 1 decimal?
Anyway, assuming all values (regardless of length) include 1 decimal:
If the number value is a general number value ....
Format(CDbl([TextNumber]/10),"#,###.0")
If the number value is to be used as currency value ....
Format(CCur([TextNumber]/10),"#,###.0")