using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var threshold = new PressureData { Time = .45 };

        var pl = new List<PressureData>() {

            new PressureData { Time = .1 },

            new PressureData { Time = .2 },

            new PressureData { Time = .3 },

            new PressureData { Time = .4 },

            new PressureData { Time = .5 },

            new PressureData { Time = .6 },

            new PressureData { Time = .7 }

        };

        var pdc = Comparer<PressureData>.Create((x, y) => x.Time.CompareTo(y.Time));

        var index = pl.FindIndexOfFirstValueLessThan(threshold, pdc);

        Console.WriteLine($"pl[{index}] = {pl[index].Time}");

    }

    public class PressureData

    { 

        public double Pressure { get; set; } 

        public double Time { get; set; }

    }

}

​

public static class ListExtensions

{

    public static int FindIndexOfFirstValueLessThan<T>(this List<T> sortedList, T value, IComparer<T> comparer = null)

    {

        var index = sortedList.BinarySearch(value, comparer);

        // The value was found in the list. Just return its index.

        if (index >= 0)

            return index;

        // The value was not found and "~index" is the index of the next value greater than the search value.

        index = ~index;

        // There are values in the list less than the search value. Return the index of the closest one.

        if (index > 0)

            return index - 1;

        // All values in the list are greater than the search value.

        return -1;

    }

}