I assume that you have implemented the IDataGridViewEditingControl to host the Textbox inside the DataGridView control. so in that case you might have also derived the TextBox control as well.
You just need to override On TextChanged event and parse the text. it will give you updated vlaue.
Please refer the below code block.
protected override void OnTextChanged(EventArgs e)
DataGridViewColumn dataGridViewColumn = this.dataGridView.Columns[this.dataGridView.CurrentCell.ColumnIndex];
ISwitchRowStatement swtRowStatement = this.dataGridView["ColRow", this.dataGridView.CurrentCell.RowIndex].Tag as ISwitchRowStatement;
if (float.TryParse(this.Text, out value))
this.dataGridView["ColumnHystersis", this.dataGridView.CurrentCell.RowIndex].Value = value;
Please post the code if above solution is not working for you.