var list = project.Lists["list"];//тут список с исходными числами
list.AddRange(list.GetItems("all",true).Select(x=>{x=x.Contains('.')?$"{x}00":$"{x}.00";return x.Length>4?x.Truncate(4):x;}));
List<string> list = new List<string> ()
{
"1.22",
"1.01",
"2",
"40.1",
"1.1",
"5.66"
};
foreach(var l in list)
Console.WriteLine($"{Math.Round(double.Parse(l.Replace(".",",")), 2):n2}");