Here's one way:

declare

@input table(fl float)

--

insert

@input(fl)

select

123456789000 union all

select

12345678900 union all

select

1234567890 union all

select

123456789 union all

select

12345.6789 union all

select

1234.56789 union all

select

123.456789 union all

select

12.3456789 union all

select

1.23456789 union all

select

0.123456789 union all

select

0.0123456789 union all

select

0.00123456789 union all

select

0.000123456789 union all

select

0.0000123456789

@sigfigs tinyint

@sigfigs = 0

@sigfigs < 11

fl float_value, @sigfigs sigfigs

floor(fl/power(cast(10 as float),floor(log10(fl))+1-@sigfigs))

power(cast(10 as float),floor(log10(fl))+1-@sigfigs) new_value

@input

select @sigfigs = @sigfigs + 1